1. Field of the Invention
Aspects of the present invention relate to a light sensing circuit, a method of controlling the same, and a touch panel including the light sensing circuit, and more particularly, to a light sensing circuit having sensitivity constantly maintained without being influenced by the ambient temperature, a method of controlling the same, and a touch panel including the light sensing circuit.
2. Description of the Related Art
Light sensing circuits, which sense current generated by light incident on a photodiode, are used for image sensors, touch panels, etc. The brightness of the incident light may be determined, or a finger touch on a display panel may be detected, by sensing the generated current.
FIG. 1 is a circuit diagram of a conventional light sensing circuit. As shown in FIG. 1, a conventional light sensing circuit includes a photodiode D, a driving transistor Tr_dr, a switching transistor Tr_sw, an initiation transistor Tr_init, and a reset transistor Tr_rst.
The photodiode D senses light incident from the outside and generates current according to the brightness of the light. The photodiode D includes an anode connected to a first power supply having a first voltage VSS and a cathode connected to a sensing node SN. The photodiode D is reverse biased, and thus, the potential of the anode should be lower than the potential of the cathode. If light is incident from the outside while the photodiode D is reverse biased, current is generated according to the brightness of the light.
The initiation transistor Tr_init periodically applies an initiation voltage Vinit to the sensing node SN. The initiation voltage Vinit is a voltage reverse biasing the photodiode D and satisfies Vinit>VSS. A first electrode of the initiation transistor Tr_init is connected to the sensing node SN, a second electrode of the initiation transistor Tr_init is connected to a second power supply which supplies the initiation voltage Vinit, and a gate electrode of the initiation transistor Tr_init is connected to an initiation line INIT through which an initiation signal is applied to the initiation transistor Tr_init.
The driving transistor Tr_dr outputs a brightness current corresponding to the brightness of light incident on the photodiode D. In this regard, the brightness current is obtained by amplifying current generated in the photodiode D. A first electrode of the driving transistor Tr_dr is connected to a third power supply which supplies a third voltage VDD to the first electrode, and a gate electrode of the driving transistor Tr_dr is connected to the sensing node SN. In addition, a second electrode of the driving transistor Tr_dr is electrically connected to a data output line D_out.
The switching transistor Tr_sw controls current generated in the driving transistor Tr_dr to flow in the data output line D_out. A first electrode of the switching transistor Tr_sw is connected to the second electrode of the driving transistor Tr_dr, and a second electrode of the switching transistor Tr_sw is connected to the data output line D_out. In addition, a gate electrode of the switching transistor Tr_sw is connected to a light integration control line Integ to which a light integration signal is applied.
The reset transistor Tr_rst controls the data output line D_out to be periodically grounded. A first electrode of the reset transistor Tr_rst is connected to the data output line D_out, and a second electrode of the reset transistor Tr_rst is grounded. In addition, a gate electrode of the reset transistor Tr_rst is connected to a reset line Reset to which a reset signal is applied. The reset transistor Tr_rst is turned on by the periodically applied reset signal so that the potential of the data output line D_out is periodically 0V.
A method of controlling the conventional light sensing circuit will be described with reference to FIG. 1. When the initiation transistor Tr_init is turned on by an initiation signal, the potential of the sensing node SN is set to an initiation voltage Vinit. Since the initiation voltage Vinit is greater than the first voltage VSS, the photodiode D is reverse biased.
When light is incident on the photodiode D, current is generated according to the brightness of the incident light. The greater the brightness of light, the higher the current generated in the photodiode D. Since charges flow into the sensing node SN by the generated current of the photodiode D, the potential of the sensing node SN decreases.
The driving transistor Tr_dr has a low level voltage between gate and drain electrodes of the driving transistor Tr_dr since the potential of the sensing node SN decreases, and thus current corresponding to voltage between the gate and source electrodes of the driving transistor Tr_dr is generated.
When the switching transistor Tr_sw is turned on by the light integration signal, current generated in the driving transistor Tr_dr flows through the data output line D_out. An output sensing unit (not shown) may be disposed at one end of the data output line D_out to detect the level of the brightness of the incident light according to the current flowing through the data output line D_out. For example, the output sensing unit may include a capacitor. In this case, charges are stored in the capacitor by the current flowing through the data output line D_out. The brightness of the incident light may be determined by measuring voltages of both ends of the capacitor.
However, the sensitivity of the operation of the conventional light sensing circuit may vary according to temperature. Since current generated in the photodiode D may vary according to temperature, the brightness of the incident light obtained using the current flowing through the data output line D_out may not be accurately obtained.
FIG. 2A is a graph illustrating a dark current generated in the photodiode. In FIG. 2A, a horizontal axis represents a reverse voltage applied across the photodiode D, and a vertical axis represents a dark current flowing through the photodiode D. The upper curve represents current measured at 40 degrees Celsius, and the lower curve represents current measured at 25 degrees Celsius. Referring to FIG. 2A, the higher the temperature, the more current flows at the same brightness.
The potential of the gate electrode of the driving transistor Tr_dr gradually decreases in a time period during which light is applied to the photodiode D increases. Due to the gradual decrease in the potential of the gate electrode of the driving transistor Tr_dr, current corresponding to the voltage between the gate and source electrodes flows from the source electrode to the drain electrode of the driving transistor Tr_dr. However, if the voltage between the gate and source electrodes of the driving transistor Tr_dr exceeds a certain level, the current flowing in the driving transistor Tr_dr is saturated and stops increasing. These characteristics may be confirmed by the V-I curve of a transistor. Even though light with different brightness is incident on the photodiode D, the current flowing through the data output line D_out may be identical to each other.
Even though the value of the current flowing through the data output line D_out is different, an output value may be saturated according to the brightness. For example, the output of the output sensing unit may be saturated, as will be described in more detail with reference to FIGS. 2B and 2C.
FIGS. 2B and 2C are graphs illustrating resolution of the conventional light sensing circuit according to the light integration time period in a conventional method of controlling a light sensing circuit. In FIGS. 2B and 2C, a horizontal axis represents light integration time periods, and a vertical axis represents output values measured according to current flowing through the data output line D_out. The output values may be voltages of both ends of the capacitor of the output sensing unit. In this regard, an output value is measured when the conventional light sensing circuit is at room temperature in FIG. 2B, and the output value is measured when the conventional light sensing circuit is at a temperature higher than room temperature in FIG. 2C.
Referring to FIG. 2B, if the light integration time period is t1, the output value in proportion to the brightness may be measured. The greater the brightness, the higher the output value. The less the brightness, the lower the output value. Thus, the brightness of the incident light may be accurately determined.
Referring to FIG. 2C, if the light integration time period is t2, the output value is in proportion to the brightness, and thus the brightness of the incident light may be accurately determined. However, if the light integration time period is t1, the output value is saturated, and thus the output value may be the same regardless of the brightness of the incident light. At light integration time period t1, even though different currents are generated according to the brightness, the output values are the same since the current reaches the limit of the capacitor disposed at one end of the data output line D_out.
Thus, there is a need to develop a method of driving a light sensing circuit having sensitivity constantly maintained without being influenced by the ambient temperature.